1. Field of the Invention
The present invention relates to a photosensitive composition for an optical element having stable optical properties used in a liquid crystal display, and an optical element using the photosensitive composition, and a method of manufacturing the optical element.
2. Description of the Related Art
Phase difference films are widely used as phase difference controlling functional layers in liquid crystal displays. For example, for a reflecting liquid crystal display, a linearly polarizing plate and a ¼λ phase difference plate are generally used together to obtain circularly polarized light. Moreover, in a vertical alignment mode LCD that has become widely used in liquid crystal TVs in recent years, to reduce the viewing angle dependence, a phase difference film having an optical axis perpendicular to the substrate and a negative birefringence anisotropy (a negative C plate) and a phase difference film having an optical axis parallel to the substrate and a positive birefringence anisotropy (a positive A plate) are used together.
In addition, many other phase difference films such as viewing angle compensating films using discotic liquid crystals have come onto the market. Such a phase difference film, in which another phase difference plate and a polarizing plate are bonded with the optical axes at a certain specific angle to one another, is used bonded to the outside of a liquid crystal cell. Here, an adhesive used in the bonding has a different refractive index to the phase difference plate and the polarizing plate, and hence reflection of external light occurs at the bonding interface. This reflection of external light causes a decrease in the display contrast, causing a marked deterioration in the image quality.
This problem can be resolved by forming a phase difference controlling functional layer inside the cell using a liquid crystal material instead of using a phase difference film as described above. That is, a phase difference controlling functional layer can be provided by using a liquid crystalline polymer that has a glass transition point and for which the liquid crystal structure can be frozen below this glass transition point, or a polymerizable liquid crystal that has reactive groups such as unsaturated bonds in the molecular structure thereof and for which the liquid crystal structure can similarly be frozen by crosslinking the unsaturated bonds together while in a liquid crystal layer state. As such polymerizable liquid crystal materials, various materials have been proposed, for example in Published Japanese Translation of PCT Application No. 11-513019.
However, conventionally, when forming a phase difference controlling functional layer inside a cell, in the case of providing a phase difference controlling functional layer between a glass substrate and a color filter layer using a polymerizable liquid crystal material, there has been a problem that the performance of the phase difference controlling functional layer deteriorates through a heating process in the color filter manufacture. Moreover, even in the case of providing the phase difference controlling functional layer on the color filter layer, there has been a problem of the phase difference controlling functional layer cracking through a subsequent electrode establishing process in which a transparent conductive ITO film is sputtered on.
In view of the above problems, it is an object of the present invention to provide a photosensitive composition for an optical element that has stable optical properties, i.e. for which the performance of a phase difference controlling functional layer does not decrease upon heating when manufacturing color filters used in an image display apparatus such as a liquid crystal display and, and cracking or the like does not occur upon ITO sputtering in an electrode establishing process, an optical element using the photosensitive composition, and a method of manufacturing the optical element.
The present inventors considered that causes of the above problems were the crosslink density of the phase difference controlling functional layer being low, and the distance between crosslinking sites being long due to the polymerizable liquid crystal molecules being crosslinked together at terminal groups thereof, and hence the molecular alignment in the phase difference controlling functional layer being disturbed upon heating, and as means for resolving the above problems, discovered that by applying a photosensitive composition containing at least a polymerizable liquid crystal material, a photopolymerization initiator, a crosslinking agent, and a solvent onto a supporting material, irradiating with ultraviolet radiation while aligned in a liquid crystal state so as to cure, and then carrying out heating treatment, so as to crosslink through the action of the crosslinking agent between the mesogenic backbones in the cured polymerizable liquid crystal material, the heat resistance and the ITO resistance of the phase difference controlling functional layer are improved.